Cage fusion from bi-cages to tri-cages during nucleation of methane hydrate: a DFT-D simulation

Abstract

Water-cage clusters encapsulating guest molecules are the basic components of hydrate crystal structures. Herein, we investigated the fusion process from bi-cages to tri-cages to probe the nucleation mechanism at the initial stage of CH₄ hydrate formation by employing dispersion-corrected density functional theory. We found that tri-cages possess high stability by sharing three, rather than two, polygonal faces. In addition, any mono-cage combined with a nonstandard 4¹5¹⁰6² cage could achieve considerable stability regardless of which face is shared; this finding illustrates that 4¹5¹⁰6² cages are more likely to appear at the early stages of CH₄ hydrate nucleation than other nonstandard cages. We then simulated the Raman spectra of CH₄ molecules in water-cage to characterize the spectral characteristics of the CH₄ hydrate. The C–H symmetric stretching frequency of encapsulated CH₄ molecules red-shifted with increasing mono-cage size, which follows the prediction of the “loose cage–tight cage” model. The symmetric stretching vibrational frequencies of trapped CH₄ molecules in the tri-cage revealed a clear red-shift compared with those in the component mono- and bi-cages. The cage fusion process and spectroscopic properties described in this work are expected to provide new atomistic insights into CH₄ hydrates at the initial nucleation stage.